Phototube



D. G. HAINES April 14, 1936.

PHOTOTUBE Filed Oct. 25, 1933 FIG. 3

INVENTOR DONALD G. HAINES ATTQRNEY Patented Apr. 14, 1936 PATENT OFFICE2,037,075 PHOTOTUBE Donald G. Haines, Bloomfield, N. J., assignor toRadio Corporation of America, a corporation of Delaware ApplicationOctober 25, 1933, Serial No. 695,062

3 Claims.

This invention relates to phototubes and more particularly to aphotocathode or light sensitive element for such tubes.

A well known type of phototube has a concave photocathode, usuallysemi-cylindrical, and a rod anode positioned approximately at the focusor axis of the cathode. As the effective light sensitive surface of sucha cathode is the concave surface, the incident beam of light must bewithin a comparatively narrow angle and the total useful electronemission is determined by the surface area of the concave side of thephotocathode.

One object of my invention is to provide a phototube having a lightsensitive element energized by difiused incident light or by lightfalling upon it from any and all directions.

Another object of my invention is to provide a phototube cathode soconstructed that the photo emission from its entire surface is useful inthe operation of the tube.

In one embodiment of my invention the light sensitive photocathode isapproximately cylindrical, and is made of sheet metal perforated with alarge number of holes or openings of any desired shape and of a diameterseveral times the thickness of the sheet metal. These openings arepreferably so distributed that the solid metal of the cathodeconstitutes about one-half its surface. The entire surface of thisphotocathode, both inside and outside, is light sensitized by some wellknown method, such as that used to make thin film caesium oxidephotocathodes. The straight or rod anode, mounted within and at thecenter of the cylindrical photocathode, draws photoelectrons emitted bythe outside of the photocathode through the holes in the cathode so thatthe photo emission from both the inside and the outside surfaces isutilized.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, will be best understood by reference to the followingdescription taken in connection with the accompanying drawing in which:

Figure 1 is a perspective view with parts broken away of a phototubeconstructed in accordance with my invention;

Figure 2 is a perspective view of a modification of the phototube shownin Figure 1;

Figure 3 is a circuit diagram of a representative type of circuit inwhich the phototubes of Figures 1 and 2 may be operated.

The preferred phototube construction illustrated in Figure 1 as oneembodiment of my invention comprises a sealed evacuated bulb ID oftransparent glass enclosing the tube electrodes and having the usualbase I l and a reentrant stem l2 which carries an electrode assemblyconsisting of an anode I3 and a photocathode M. The anode I3 may be arod of nickel or similar metal and the photocathode I4 is considerablysmaller in diameter than the bulb and preferably made an open endedcylinder of perforated sheet metal having a large number of holes oropenings I5 I arranged in rows and distributed over all the pe- 1oriphery of the cylinder. I have obtained good results with a cylindricalphotocathode threefourths of an inch long, three-fourths of an inch indiameter, and made of sheet metal mils thick, with holes I 5 each about40 mils in diameter and having a total area of approximately 48% of thetotal photocathode surface area. The foundation metal of thephotocathode is preferably silver although silver surfaced nickel orcop'- per coated nickel with a silver coating on the copper may be used.Preferably both sides of the metal are silver surfaced so that when thephotocathode is sensitized both the inside and outside of thephotocathode Will emit photoelectrons, although a nickel strip with asilver surface on only one side may be used if a photocathode lightsensitive on only one side is desired.

The tube may be made by the methods usually employed in making the thinfilm caesium type of phototube, such as described in U. S. Patent toBainbridge, #1,901,578, March 14, 1933. After the tube has beenexhausted the caesium may be introduced by heating with a high frequencycoil the pellet l6 which contains the usual compounds for liberatingcaesium. The liberated caesium deposits on the previously oxidizedsilver surface of the cathode and makes the surface light sensitive.

Figure 2 shows a modification of the tube of Figure 1, with acylindrical highly polished anode I! of substantially the same length asthe cathode I5. The light passing through the holes l5 of thephotocathode I4 is reflected by the polished anode l1 and falls on thelight sensitive inner walls of the photocathode I4 thus substantiallincreasing the total useful photoelectron emission from the perforatedphotocathode.

In operation of the tubes shown in Figures 1 and 2, which may be eithervacuum or gas filled, the light incident on the outside solid portionsof the photocathode l4 causes an emission of photoelectrons which ingeneral are pulled through the holes l5 to the inside of thephotocathode and then to the anode l3, while light passing through theholes l5 and falling on the inner surface of the photocathode l4produces photoelectrons which go directly to the anode I3. The holes lare preferably large in comparison with the thickness of the sheet metalof the cathode so that the positive field of the anode can in effectreach through the holes 15 and pull in the photoelectrons emitted fromthe outside surface of the photocathode 14. If the holes l5 are toosmall compared to the thickness of the sheet metal of the photocathode14, the pull of the anode is not great enough to draw the electrons fromthe outside of the photocathode through the holes.

An obvious commercial use of the phototube such as shown in Figures 1and 2 is to mount the tube on a parabolic reflector, preferably at thefocus of the reflector, so that light impinging upon the reflector willbe concentrated onto the light sensitive photocathode and cause it tooperate a relay connected to produce the desired result.

Figure 3 shows a representative type of circuit in which tubes ofFigures 1 and 2 may be operated. The photocathode I4 is connected to thenegative pole of the supply battery l8 and the anode I3 is connected tothe positive pole of the battery through an impedance l9. When lightfalls upon the photocathode M photoelectrons are emitted and are drawnto the anode l3, causing a flow of current in the external circuit ofthe two electrodes. This current flows through the impedance l9 andproduces across this impedance a voltage drop which may be utilized inthe output circuit 20. For example, the circuit 20 may be connected to arelay system or to the input circuit of the first tube of a thermionicamplifier such as that used in a picture transmission or televisionsystem. A condenser 2| may be used to block from the phototube anyvoltage from the output circuit, such as the grid bias voltage commonlyused on the first tube of an amplifier, so that such voltage will notinterfere with the operation of the phototube.

My invention may be utilized in the type of combination phototube andamplifier disclosed in U. S. patent application of Herbert Nelson#690,045, filed Sept. 19, 1933, and assigned to the same assignee as thepresent application in which case the anode and other amplifier elementswill be placed within the cylindrical photocathode as shown in thatapplication.

The embodiments of the invention hereinabove disclosed are merelyillustrative and that many changes may be made without departing fromthe scope of the invention which is limited only by the appended claims.

What is claimed as new is:

l. A phototube comprising, a perforated light sensitive photocathode,and a light reflecting anode surrounded by said cathode for causingmultiple reflection of light projected through said cathode and forcollecting the photoelectrons emitted from said cathode due to theprojected and reflected light.

2. A phototube comprising a tubular photoelectric cathode with all ofthe inner and all of the outer surfaces sensitized to light and havingin each quarter of its periphery an aperture through its wall greater indiameter than the thickness of said wall to admit light to the interiorof said cathode, and a tubular sheet metal anode mounted inside andcoaxial with said cathode and having a light reflecting outer surface tocause multiple reflection of light projected through said cathode.

3. A substantially non-directional phototube comprising an evacuatedenvelope enclosing a tubular photoelectric cathode spaced from the wallsof said envelope with light sensitized surfaces and having in eachquarter of its periphery a plurality of apertures through its wallgreater in diameter than the thickness of said wall, and a cylindricalelectron collecting electrode inside said photocathode to receiveelectrons and with an exposed light reflecting outer surface to receiveand reflect light entering said apertures.

DONALD G. HAINES.

